1. Field of Invention
The present invention relates to an absorber that is capable of efficient absorption of SiF.sub.4 and/or BCl.sub.3 present in gases that are discharged from semiconductor fabrication plants, ceramic producing plants and other facilities.
2. Prior Art
Gaseous SiF.sub.4 and BCl.sub.3 are discharged, in small amounts though, from the steps of IC or LSI fabrication or from the sintering step of ceramic production. These gases are injurious to the human body and are currently removed by either the wet or dry absorption process. The wet method involves the use of water or an aqueous alkali solution with which the effluent gas is treated so as to absorb the injurious components. This method is so popular in the art of emission treatment that it can be implemented quite easily. In addition, this method is very efficient since SiF.sub.4 and BCl.sub.3 gases are highly reactive with water. Rapid absorption of these gases by water is illustrated by the following reaction schemes: EQU SiF.sub.4 +2H.sub.2 O.fwdarw.4HF+SiO.sub.2 ( 1-1) EQU 2HF+SiF.sub.4 .fwdarw.H.sub.2 SiF.sub.6 ( 1-2) EQU BCl.sub.3 +3H.sub.2 O.fwdarw.H.sub.3 BO.sub.3 +3HCl (2-1).
However, the wet process has the following disadvantages: it involves high initial and running costs not only for circulating the absorbing liquor but also for controlling the pH and the concentration of the resulting salts; the impurities in the emission will react with the absorbing liquor to form solid matter that either clogs the equipment or increases the concentrations of salts in the absorbing liquor, which leads to scaling or retardation of the intended reactions; and it is necessary to treat the liquid waste resulting from the wet process.
The dry process has been proposed with a view to solving these problems of the wet process. The dry process is classified as the full-dry type which employs a solid alkali agent per se as an adsorbent (see Unexamined Published Japanese patent application No. 61619/1986) or as the semi-dry type which employs a diatomaceous earth carrier impregnated with an aqueous alkali solution (see Unexamined Published Japanese patent application Nos. 128146/1983 and 187335/1985). In either type of the dry process, an adsorbent is packed in a column and the intended treatment can be accomplished more easily and with smaller equipment required in comparison with the wet process. However, the reaction occurring in the full-dry method is so slow compared with the wet process that an unduly large amount of adsorbent must be packed in the column. Furthermore, the water that forms as a result of reaction will sometimes cause agglomeration of the particles of the adsorbent, which leads to an increased resistance to gas permeation. In these respects, the semi-dry method seems to be more efficient but the water retention of the carrier such as diatomaceous earth is not large enough to allow for the increase in the contents of water and an alkali agent that can be incorporated in unit volume of the adsorbent, and this eventually leads to an increased use of the latter. In addition, the water formed as a result of reaction cannot be absorbed rapidly enough to render the semi-dry method completely free from the problem of increased resistance to gas permeation.